The invention disclosed relates to an electronic device for controlling the supply of electrical power to resistance welding equipment.
Such a device comprises a frequency converter that supplies the primary winding of the welding transformer with an output voltage having waveshape that is characterized by a pulse train of constant amplitude, and an electronic regulator which is designed to compare a feedback signal, proportionate to amplitude and frequency of the welding current, with its own preset references; on the basis of such comparison, the converter is regulated in such a way that the voltage supplied to the primary winding of the transformer will be of amplitude and frequency of such as produces the prescribed welding current.
In the art field of resistance welding equipment, in particular, such as is employed for lap welding the longitudinal seams of metal box components and the like, the system by which electrical power is supplied to the welding rollers must be capable of producing welded joints that are free from flaws, and possess dimensional and qualitative features which will remain constant in the long term and respond to given specifications, as far as is feasible.
The prior art embraces various power supply devices designed to achieve the aforementioned objects. In one such device, power is supplied by exploiting alternating current with a sine wave shape and raising the frequency in measure such as to match the welding speed required, rotary or static frequency converters often being employed for the purpose. This first type of device is beset by various drawbacks however, amongst which, that of a less than regular transmission of power, accompanied by the need to employ current characteristics, such as the operating frequency, that cannot be allowed to drop below a given threshold; this in turn occasions excessive overheating of the equipment and, clearly enough, somewhat high power consumption.
A second type of device utilizes alternating current not with sine wave, but with what is practically a square wave shape. Square wavelengths offers marked advantages over sine waveshape, since power can be transmitted to the weld material in a more regular fashion, and lower values of current amplitude and frequency can be adopted, thereby gaining a strong attenuation of the equipment's tendency to overheat, and cutting power consumption attributable to the welding process and to cooling of the equipment.
This second type of device demands particularly cumbersome, heavy duty equipment however, and directly related to such a drawback is the specific impediment that, in order to obtain optimum slope control on the square wave welding current, it becomes necessary to utilize two or more frequency converters the alternating output voltages of which will be integrated in phase so as to provide square wave current exhibiting a maximum of slope. In effect, if the slope is sufficiently steep, then inversion of the welding current will come about in a negligible period of time, and a much lower operating frequency can be employed.
Significant progress has been achieved with a third type of device that consists substantially in a converter, connected to the power source, that provides an output pulse train exhibiting constant amplitude and split into alternating positive and negative groups of pulses; the current produced reaches the requisite amplitude during each positive or negative half-period, and in the shortest time possible. Such a device comprises an electronic regulator which compares amplitude and frequency of the welding current with its own preset reference values and, on the basis of such a comparison, regulates the converter in such a way that voltage supplied to the transformer will be of amplitude and frequency such as can produce the prescribed welding current.
With a device of this type, it becomes necessary to operate with the secondary winding of the transformer handling rather high voltage, since the substantially square wave output current from the converter features a steep slope characteristic. Thus one is able to avoid the majority of problems connected with flawed weldments, and with the square wave output, operation at low frequency reduces the tendency of the equipment to overheat.
Nonetheless, there are drawbacks connected with this third type of prior art device, one of which stems from the fact of operating with high voltage levels passing through the secondary winding of the transformer. The need arises for large transformers, and as a result, for enlargement of the converter stage, since current put through the primary winding needs to be of a somewhat high order. There are also limits imposed on welding speed in the case in point since, from a given speed upwards, problems with flaws will reappear; this means that, even operating with high voltage across the secondary winding of the transformer, one is necessarily obliged to step up the operating frequency and thus to accept the overheating-related drawbacks that this entails.
The essential objects of the invention disclosed are those of overcoming the various drawbacks described, solving the problems connected with resistance weld flaws comprehensively, and providing the facility of operating with low voltage levels passing through the secondary winding of the transformer so that use can be made of standard size converters and transformers.
Advantages provided by a device according to the invention are:
the elimination of overheating problems caused by eddy currents, which are reduced to a bare minimum quantifiable at approx 25-30% of the levels in prior art devices;
maximum economy in power consumption attributable to welding and cooling alike, the only dissipation occurring through Joule effect and welding current losses;
improved weld quality, as current is supplied to the welding rollers more regularly and smoothly, and any welding speed can be adopted since the need no longer exists to step up frequency, operating frequency being sufficiently high already, and variable according to requirements.